It is well known to produce carbon monoxide by a steam reforming process in which a hydrocarbon feed stock, such as natural gas, refinery gas, liquefied gas or naphtha, is treated with water vapor at elevated temperatures in contact with a catalyst and is thus converted to a gas which is rich in hydrogen and carbon monoxide. When the product gas has been cooled down, the carbon dioxide and water vapor still contained therein are removed by scrubbing with a suitable material and the gas is separated into carbon monoxide and hydrogen components at low temperature.
It is also known to improve the yield of carbon monoxide by recycling the carbon dioxide which has been scrubbed from the primary reformer product, back to the primary reformer, see for example, U.S. Pat. No. 3,988,425 to Jockel. The product obtained after removal of the carbon dioxide is often referred to as "synthesis gas" since mixtures of carbon monoxide and hydrogen are useful in synthesizing organic compounds. Methods for producing synthesis gas are described in U.S. Pat. No. 2,198,553 to Roberts, U.S. Pat. No. 2,485,875 to Gorin, et al., U.S. Pat. No. 2,496,342 to Gillespie, U.S. Pat. No. 4,175,115 to Ball, et al. and U.S. Pat. No. 4,316,880 to Jockel.
For many industrial processes, a high purity source of carbon monoxide is required. In most known synthesis gas production processes, however, a reaction product is obtained from a primary or secondary reformer which has a relatively high level of methane, which is the usual residual hydrocarbon contaminant. The usual method for the production of high purity carbon monoxide involves the use of cryogenic distillation wherein the carbon monoxide and methane are cryogenically separated from the hydrogen by liquefaction or scrubbing with liquid methane and the carbon monoxide and methane are subsequently separated from each other by distillation. The presence of relatively high levels of methane in the synthesis gas obtained from the primary or secondary reformer is not important since the methane is removed during the cryogenic separation step. The use of cryogenic distillation, of course, is an extremely expensive process for providing a high purity source of carbon monoxide.
It is also known to add oxygen to the feed stream into the reformer used to provide carbon monoxide and hydrogen from a hydrocarbon feed stock. U.S. Pat. No. 2,701,757 to Riblett, for example, indicates that if the oxygen to carbon ratio is varied a methane content in the effluent stream of from 0.5 to 5% can be obtained. A level of methane of 0.5%, however, is still tho high to provide a high purity carbon monoxide product. U.S. Pat. No. 4,891,950 to Seufert describes a means for controlling a synthesis gas process wherein oxygen and natural gas are fed to a synthesis gas generator. The Seufert patent utilizes methane sensing means and pressure sensing means connected to the conduit containing separated carbon monoxide for use in the control system. The Seufert patent does not indicate what the methane level is of the carbon monoxide product that is obtained.
U.S. Pat. No. 2,700,598 to Odell describes a process for producing synthesis gas whereby the uniformity of the temperature distribution within a primary reaction zone is obtained. In the method of the Odell patent, hydrocarbons and steam are introduced into one end of a vertically packed column and are passed longitudinally through an annular space in the column where primary reforming of the hydrocarbons takes place. The hydrocarbons are then passed from the bottom annular space into and through a centrally and axially disposed inner tube within the column. Oxygen, air, steam or a mixture thereof may be introduced into the hydrocarbons as they pass from the annular space in the column into the inner tube.
U.S. Pat. No. 4,854,943 to Voeste, et al. describes a process of producing a gas which is rich in carbon monoxide by catalytic cracking of gaseous or vaporized hydrocarbons. A carbon monoxide product gas is obtained without formation of soot even though water vapor is not added or is not added in a substantial amount. The method of the Voeste patent utilizes an oxygen-containing gas which is introduced to the combustion zone where the hydrocarbons are reformed. The oxygen-containing gas is added to the combustion zone at a rate which corresponds to twice to ten times the stoichiometric oxygen demand required for the reaction. The method of the Voeste, et al. patent produces a reformed product having less than about 0.3% methane in the reaction product. While this level of methane is lower than is obtained by many synthesis gas reforming processes, the level is still not low enough to provide a high purity carbon monoxide product. In addition, the levels of oxygen required in the Voeste, et al. patent make the process undesirable from an economic standpoint.
U.S. Pat. No. 3,120,431 to Carton, et al. describes a method for producing synthesis gas wherein the temperature of the catalyst tube walls are maintained at a substantially constant value throughout the height of the heating furnace. In the method of the Carton, et al. patent a mixture of hydrocarbons and an oxidizing gas, selected from steam and carbon dioxide, are caused to flow upwardly inside a plurality of vertical tubes over a catalyst for the reforming reaction. Fuel is burned at several successive levels near the tubes to form a stream of hot combustion gases flowing around the tubes in the same direction as the feed stock mixture passing over the catalyst. An indirect heat-exchange relationship with the feed stock to reform the feed stock is obtained. The method also involves adjusting the heat inputs of the combustion gas for maintaining the temperature of the outside surface of the tubes at a substantially constant value over the entire length of the tubes in which the catalytic reforming is being executed.
PCT Application WO 87/06221 to Egglestone describes a method and apparatus for the production of synthesis gas utilizing primary and secondary reforming which utilizes the available heat of the effluent from the secondary reformer to provide heat for the primary reformer. The process includes the steps of (a) feeding hydrocarbon-containing gas and steam to a primary reforming zone containing a primary steam reforming catalyst under reforming conditions wherein the hydrocarbon gas is partially reformed to produce a primary reformer effluent, (b) feeding the primary reformer effluent and oxygen-containing gas to a secondary reforming zone containing secondary reforming catalyst under reforming conditions wherein a secondary reformer effluent is produced, (c) passing the secondary reformer effluent to the primary reforming zone as indirect heating medium, and (d) removing the secondary reforming effluent from the primary reforming zone and recovering the raw synthesis gas. There is no teaching in the Egglestone patent, however, of the use of high levels of import gas to provide high purity carbon monoxide product without the need for cryogenic separation.
Accordingly, it is a principal object of the present invention to provide a method for the manufacture of high purity carbon monoxide.
It is another object of the present invention to provide a method for producing high purity carbon monoxide which does not require cryogenic purification processes for hydrocarbon contaminants.
It is a further object of the present invention to provide a method for producing high purity carbon monoxide by an efficient continuous process which converts substantially all of the hydrocarbon feed to carbon monoxide, carbon dioxide and hydrogen without leaving any substantial residue of hydrocarbon impurity.
These and other objects will become more apparent from the following description and the accompanying claims.